Developmental Cerebellar Pathology in Mouse Models of SCN2A Premature Termination Codon Variants

Autism spectrum disorder is a neurodevelopmental disorder with both genetic and environmental contributors. SCN2A , the gene which encodes the alpha subunit of the voltage-gated sodium channel Na _v 1.2, is a known monogenetic risk factor for autism spectrum disorder. The cerebellum is frequently implicated in autism spectrum disorder and other neurodevelopmental disorders but has largely been unexplored in relation to SCN2A variants, especially from a developmental perspective. Na _v 1.2 is highly expressed within the cerebellum, specifically within cerebellar granule neurons, where it helps to drive action potential generation and propagation. Cerebellar granule neuron activity and maturation is crucial for shaping the development and morphology of the rest of the cerebellar cortex. Here we investigated early-postnatal cerebellar development in two mouse models carrying patient-derived SCN2A premature termination codon variants, Scn2a- p.Y84X and -p.R1627X. Overall, both Scn2a premature termination codon variant mouse lines displayed largely normal physical development and unaffected non-cerebellar developmental milestones. Scn2a ^Y84X/+ mice, but not Scn2a ^R1627X/+ mice, demonstrated alterations in cerebellar-driven motor behaviors, specifically faster performance in the surface righting reflex and cliff avoidance compared to wildtype littermates. Coinciding with the behavioral findings, Scn2a variants had divergent and age-dependent effects on cerebellar glutamatergic presynaptic marker expression, indicating that some, but not all, Scn2a premature termination codons impair glutamatergic synapse development. Both Scn2a variants exhibited changes to cerebellar cytoarchitecture, such as reductions in Purkinje cell density and soma size, which was more prominent in Scn2a ^Y84X/+ mice, and faster migration of cerebellar granule neurons from the external granule layer to the internal, indicating a possible shift in the timing of cerebellar maturation. Our results situate the cerebellum as an early site for SCN2A pathophysiology and establish that cerebellar consequences of SCN2A premature termination codon variants may be position- and age-dependent, suggesting that influences beyond simple heterozygous loss of Nav1.2 drive phenotypes. Our characterization of how Scn2a premature termination codon variants differentially impair cerebellar development may help explain the heterogeneity of clinical presentations of SCN2A loss of function variants and potentially inform upon the timing of therapeutic intervention.